The present invention concerns a control method for a steam turbine plant comprising supply pressure control, the method to be used in particular in conjunction with a turbine plant drawing its steam from a boiling water reactor. The invention further concerns control apparatus for performing the method, to be used in conjunction with a steam turbine plant, comprising a supply pressure control device and a bypass valve system, as well as a control valve system for controlling the rate of steam flow through the turbine.
The control of the supply pressure, i.e. the pressure control of the supply steam before it reaches the turbine, offers advantages primarily in the steady state operation at essentially constant load and represents the usual mode of operation, in particular in the case of turbine plants having as its steam generator a boiling water reactor. The boiling water reactor is here mentioned as an example of a steam generator operating by way of a simply closed circuit of steam supplied directly to the turbine, wherein the rate of its recirculated steam flow should be held constant during normal operation. Such a supply pressure control device is not suitable for handling the rapid load changes arising in the usual steam turbine plants with electrical generators. In the case of a boiling water reactor or a steam generator similar to the one mentioned before, the supply pressure and the flow rate of recirculated steam is maintained by means of a bypass-valve system, which, in case of load decreases, carries the excess steam to the condenser, by bypassing the turbine.
On the other hand, steam turbine plants with conventional steam generators, whose rate of steam flow and supply pressure can be controlled with comparatively little delay times and adjusted to changing operating conditions, are usually operated by means of the well known rpm-power output control device, i.e. with an rpm-control device comprising a subordinated power output control device. This generally known control system is characterized in its steady state operation by curve characteristic lines of rpm versus power output influenced by two parameters, namely by the slope of the line, which corresponds to the static setting, and the rpm at a predetermined reference power output, as for instance at no load or at rated load. In the "steady state" control state the operating point determined by the two control quantities "rpm" and "power output" lies on the appropriate characteristic line, not considering a possible remaining control deviation, which might occur in the case of proportional control; however, the location of a transient operating point on the characteristic line is determined by the effective disturbances of rpm, frequency or load. In the limiting cases of impressed rmp (generator connected to a network of constant frequency), on the one hand, and of impressed load, on the other, the turbine power output and the turbine rpm corresponding to a specific pair of parameters establishing the characteristic line are forcibly determined. To keep the rpm constant thus requires, in general, an adjustment of the characteristic line parameters, generally of the rpm at no-load or rated load (frequency control).
The rpm-power output control and the frequency-power output control are basically suitable for handling the various operational states occurring in practice and in particular the changes in load; however, their characteristics are not necessarily in harmony with the realities of a supply pressure control.